1. Field of the Invention
The present invention relates to electrophotography and, more specifically, to a multi-layered electrophotographic element.
2. Description of Related Art
In electrophotography, an image is formed on an electrophotographic element generally comprising a photoconductive layer on a substrate by first providing the surface of that layer with a uniform electrostatic charge and then exposing it imagewise to light. The imagewise exposure causes the areas struck by light to become conductive and the charge in this area is discharged, while the charge in the non-exposed areas remains, forming an electrostatic latent image. This latent image is rendered visible, for example, by depositing finely divided electroscopic toner particles onto the imaged layer surface, the toner particles being attracted by the remaining charge. In direct electrophotography, the image thus rendered visible is fixed locally onto the electrophotographic element, e.g. by heat and/or pressure. In indirect electrophotography, the image formed on the photoconductive element is first transferred to and then fixed onto a support, usually of plain paper. Thereafter, any toner particles remaining on the photoconductive layer surface are cleaned therefrom to make the element suitable for a subsequent copying cycle.
The electrophotographic element in practice may comprise a radiation-sensitive charge generating layer applied to a conductive support. Alternatively, it may comprise a plurality of layers including a radiation-sensitive charge generating layer and a charge transporting layer applied thereto. Such multi-layered electrophotographic elements are described, for example, in U.S. Pat. Nos. 3 713 820, 3 725 058, 3 824 099, 3 837 851, 3 839 034 and 3 898 084.The radiation-sensitive compound or compounds in such a charge generating layer may be of an inorganic or organic nature. When inorganic material is used, it generally is present either in the form of particles dispersed in a binder or in the form of a homogeneous film obtained, for example, by vapor deposition. Selenium is the inorganic material most commonly used. When organic material is used, it may, for example, be present in the form of a film-forming organic polymer, such as, for example, a polyvinyl carbazole or polyvinyl pyrene, or in the form of finely divided pigment particles, such as, for example, bisazo pigments, of which Phenelac Blue and its derivatives are among the best known, which are dispersed in an organic binder.
Such pigment-binder layers, however, have exhibited a number of disadvantages. Hence, processes have been proposed for preparing charge generating layers in which the radiation-sensitive compound is present in a molecularly divided form instead of in the form of pigment particles. The advantage of the former is that the layers can be made much thinner and smoother than pigment-binder layers and are also better in respect of their charge transport and resolving powers. Furthermore, the grinding operations required for the preparation of pigment-binder layer compositions can be dispensed with.
Charge generating layers containing radiation-sensitive compounds in molecularly divided form are described, for example, in U.S. Pat. Nos. 4 123 270 and 4 286 040 and in U.K. patent No. 1 172 355. Recently organic photo generating pigments such as perylene pigments, bisazo pigments, quinone pigments, and phtalocyanine pigments have been used. Such pigments can be evaporated fairly easily in a vacuum and subsequently can be deposited on a suitable substrate to obtain thin extremely homogeneous layers.
When, because of their associated advantages, it is desired to use such layers in the thinnest possible form, e.g. at a thickness of not more than 1 to 2 .mu.m, they must be provided with a top layer because of the vulnerability of the extremely thin layers to abrasion, and in order to be able to bring them to the required charge level during uniform charging. The top layer to be used must also permit the transport of charge carriers, usually holes, which are formed in the charge generating layer during the imagewise exposure.
Perylene pigments are widely used in charge generating layers in photoconductive elements for electrophotography. The currently used perylenes have properties which make them suitable for a wide variety of photoconductors. However, there remains a need for improved photoconductive elements in, for example, electronic printers, such as laser printers, where sensitivity in the near infrared is often required. Also, there remains a need for photoconductors with an improved photosensitivity, low residual charge after illumination, a rapid discharge rate and low dark decay.